Mobile offshore drilling units and offshore production platforms are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor.
The various types of mobile offshore drilling units include so-call bottom-supported rigs which rest on the seafloor. Jack-up drilling units are typical examples of bottom-supported units; they comprise a hull and a number of legs adapted to be lowered towards the sea floor. Such jack-up units may thus be towed towards their desired off-shore location with the legs in a raised position. Once the unit is at its intended position the legs are lowered and brought into contact with the sea floor. Further lowering of the legs relative to the hull causes the hull to be elevated out of the water. Many jack-up drilling units have the drill floor and well centre positioned on a cantilever system that can be extended horizontally outwards relative to the hull of the jack-up unit, thus allowing the well centre to be positioned outside the periphery of the unit defined by the hull of the unit.
Offshore production platforms used for extracting oil or gas from a production well are frequently fixedly installed during longer periods. They frequently operate a plurality of wells.
When an oil or gas well is no longer economical or if there is some problem with the well which means that production is no longer possible or that well integrity has been compromised in some way, or for other reasons, the well may be abandoned. It is common practice to plug the well before abandoning it, e.g. to prevent seepage of hydrocarbon product from the well. This can also apply to water injectors, i.e. bores which have been drilled in order to pump water into a reservoir to increase bottom-hole pressure. Similar processes may also be relevant for drilling new wells, or so-called “slot-recovery” where the upper part of the well is re-used by plugging the lower part and side-tracking thus creating a new well using same “slot” on the production platform.
Commonly, plugging may be achieved by injecting a settable substance or medium, e.g. cement, into the well. A well will normally have production perforations, that is to say apertures in a well liner or casing through which hydrocarbon product enters from the rock formation and travels to the surface. During plug and abandonment operations it is common to seal (“squeeze”) production perforations with cement or another settable medium which may then form a permanent barrier to prevent flow through the perforations and/or across the zones and potentially out of the well. Alternatively or additionally, at least a part of the downhole equipment, such as casings, production tubing, etc., is often pulled from the well.
The process may further involve pumping mud or other high-density fluids into the well in order to stabilize or “kill the well” and, in particular to provide sufficient pressure so as to prevent carbohydrates from rising out of the well.
The process may further involve pumping a surfactant liquid, known as a “spacer”, into the well. The purpose of the spacer is to remove oil residues from the internal surface of the well casing and/or liner and rock matrix making them “water wet” (allowing better adhesion by cement) in addition to limiting cross-contamination between liquids/cement. Commonly, immediately following the spacer, cement is pumped down the well to occupy the part of the well casing and/or liner where perforations are to be squeezed. When sufficient cement has been pumped down, more spacer liquid and possibly other liquids may be pumped down the well in order to place the cement at its final designed location.
The plug and abandonment process may be a complex and costly process, and it is generally desirable to reduce the time required for performing the process, in particular when multiple wells are to be processed at the same site.
WO 2012/144952 discloses a multi-functional jack-up for decommissioning an offshore platform. In particular, this prior art jack-up unit comprises a mobile crane and a tubular-handling crane mounted to a cantilever platform of the jack-up unit and arranged to be operable independently from each other. This prior art jack-up unit aims to introduce flexibility during the well plugging and abandonment by utilizing the multifunctional jack up as a tender assist vessel which assists the offshore platform during the plugging and abandonment operation. In addition to providing assistance to the offshore platform, the multifunctional jack-up is able to perform decommissioning of the offshore platform itself.
However, it remains desirable to further improve the efficiency of the plug and abandonment process or similar processes performed on multiple wells.